Recently attention focuses on a so-called white-color Light Emitting Device (LED) in which a yellow phosphor such as YAG:Ce is combined with a blue LED to emit white-color light by single chip. Conventionally, the LED emits red, green, or blue light in monochromatic form, and it is necessary that the plural LEDs emitting monochrome wavelengths are driven in order to emit the white-color light or intermediate-color light. However, currently the combination of the light emitting diode and the phosphor removes the trouble to be obtain the white-color light with a simple structure.
An LED lamp in which the light emitting diode is used is applied to various display devices of a mobile device, a PC peripheral device, an OA device, various switches, a light source for backlight, and a display board. In the LED lamps, there is a strong demand for high efficiency. Additionally, there is a demand for high color rendering in general-purpose lighting applications, and there is a demand for high color gamut in backlight applications. High efficiency of the phosphor is required for the purpose of the high efficiency of the LED lamp, and a white-color light source in which a phosphor emitting blue excitation light, a phosphor excited by blue light to emit green light, and a phosphor excited by blue light to emit red light are combined is preferable to the high color rendering and the high color gamut.
The high-power LED generates heat by drive, and generally the phosphor is heated up to about 100 to about 200° C. Generally emission intensity of the phosphor is degraded when the temperature rise is generated. Therefore, desirably the degradation of the emission intensity (temperature quenching) is hardly generated even if the temperature rise is generated.
A sialon phosphor can be cited as an example of a phosphor that emits the green light when excited by blue light, and the sialon phosphor is properly used in the LED lamp. According to the sialon phosphor, the light emission is highly efficiently obtained with the small temperature quenching. Therefore, the high-efficiency, high-color-rendering, small-color-shift light emitting device is implemented by use of the sialon phosphor.
However, there is also a demand for further high efficiency in the light emitting device in which the sialon phosphor is used.